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Visibilities
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 Each of the four large drum filters cleans 150 m³ per hour Variations Visibility depends on the lake's levels of nutrients. We have to feed the thousands of fish that live in the lake - this means we are adding nutrients. On top of this we get influx of dust and so on. Service divers regularly remove the forming sediment, and the installed filtration systems clean more than 9,000m³ of water every day. Even so, variations in visibility cannot be avoided. Depending on temperature this happens over periods of 1-2 weeks, but can also flutuate by several metres during one day. The ability of divers is an important factor. 100 good scuba divers will hardly affect visibility, wheras a just a few who can't control their buoyancy swirl up a lot of sediment. Beginners are only allowed into the lake in company of experienced divers - but evern highly qualified scuba divers scuttle through the lake as if weighed down by 50kg of lead... Visibility in the caves is usually 20-60% better than in open water, mainly due to: Less light = less algae The dim light in the caves supresses algae growth Less sediment Because the caves are mostly closed from above, they get much less silt. What we do to improve visibility: An elaborate vacuuming system is installed in the lake. Also, service divers suction sediment from the corners. A strong influx of pollen has caused a 20cm layer of sediment to form in a few days. Without the constant work of the service divers and without the elaborate filtration system visibility would be less than one metre in the summer.
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Evaluation of Visibility
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